Manager: Telephone Features

Group 3 Fax is the common standard for fax transmission over analog and digital (TDM) phone lines. However, the sending of fax calls over IP lines is not normally possible or proves unreliable due to the distortion caused during encoding of the audio signal.

Within an IP Office Small Community Network (SCN), an IP Office proprietary protocol (Fax Transport Support) allows fax calls at one IP Office to be sent to another IP Office. However, this method is only supported between IP Offices.

T38 Fax Relay is a public set of protocols that allow fax calls to be reliable transported over IP connections that have a T38 Fax gateway at each end. For IP Office 5.0, IP500 systems with an IP500 VCM card can act as a T38 Fax gateway.

Both IP Office SIP extensions and IP Office SIP lines can be configured for T38. They can then be used as the point at which fax calls are sent and or received. Each fax call uses a VCM channel. The SIP line or extension must support Re-Invite.

In addition, the existing IP Office Fax Transport Support can still be used to transport the faxes over an IP Office SCN. That includes faxes being made or received via a T38 SIP line or extension. The conversion from T38 to Fax Transport Support uses two VCM channels.

Scenario 1:
In this scenario, the SIP provider supports T38. By configuring the IP Office's SIP line for T38 operation, the analog fax machine attached to the IP Office can make and receive fax calls via the SIP provider. In this scenario, on the IP500, 1 VCM channel is used for each T38 fax call.

Scenario 2:
This scenario is similar to the previous. However, fax calls via the SIP line are also transported across the Small Community Network to a fax machine attached to another IP Office in the network. In this scenario, on the IP500, 2 VCM channels are used for the T38 fax call.

This method is also significant in that it allows non-IP500 systems running IP Office 5.0 to make use of SIP lines for fax.

Scenario 3:
In this scenario, an ATA (Analog Telephone Adaptor) that supports T38 is used to connect the fax machine as a SIP extension on the IP Office.

System Requirements for T38

Outgoing Fax Calls

When sending a fax via T38, the call must be correctly indicated as being (or potentially being) a fax call. This can be done in 2 ways.